This application relates to the art of thermal switches and, more particularly, to thermal switches that use a contact member of temperature sensitive shape memory alloy for making and breaking a circuit. The thermal switch of the present invention is particularly applicable for use in batteries and will be described with specific reference thereto. However, it will be appreciated that the invention has broader aspects and can be used in other devices.
Shape memory alloys can be used to replace bimetals in thermal switches. A switch contact arm of shape memory alloy has a deformed shape at normal temperatures and changes to a recovered shape at elevated temperatures. These changes in shape of a shape memory alloy contact arm provide different contact arm positions that can be used to open and close an electrical circuit.
Electrical switch components of shape memory alloy have been incorporated in batteries to open the battery circuit when the battery overheats. It is difficult to incorporate a switch component of shape memory alloy into the battery construction in a manner that provides reliable operation and protects the shape memory alloy from the corrosive battery environment. Therefore, it would be desirable to have a self-contained unitary sealed thermal switch assembly that can be used with batteries and other devices in a convenient and reliable manner.
Thermal switches for use in preventing thermal runaway in batteries must have a very low resistance so they will not be too wasteful of energy during normal charging and discharging of the battery. It is difficult to achieve low resistance with a current carrying actuator of shape memory metal. For protecting batteries against thermal runaway, it also is desirable that the protective switch function as a one-shot fuse that will not reclose once it has opened.